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Nested Geometrically Decoupled Triple-Tuned Volume Coil for 1H, 23Na and 31P at 4.7T1Biomedical Engineering, Texas A&M, College Station, TX, United States, 2Electrical and Computer Engineering, Texas A&M, College Station, TX, United States, 3Electrical and Computer Engineering, Texas A&M, C, TX, United States
Synopsis
The use of traps as well as the use of switching circuitry to develop multinuclear coils is well established. However, it is well known that the use of traps introduces undesired losses to one or more nuclei in the structure while switching eliminates applications requiring true simultaneous imaging. As a result, our group developed a triple-tuned volume coil that solely uses geometric decoupling using only two structures. The coil demonstrated homogeneous fields with sufficient decoupling between the structures to acquire multinuclear NMR data.
Introduction
The application of multinuclear NMR experiments brings the need for specialized coils to enable sensitivity to these nuclei. Importantly there is the need to be able to image without moving the sample for colocalization of X-nuclei data to a predetermined region of interest using standard hydrogen imaging techniques1. The use of traps is a well-established technique however, it is known to introduce undesired losses to one or more nuclei in the structure2,3. Switching circuits are another commonly used approach either through the use of PIN diodes or MEMS circuits to create multinuclear coils while reducing the loss in Q and sensitivity associated with many simultaneous multinuclear coil designs4. However, these circuit designs do not allow for true simultaneous imaging that is needed for emerging multinuclear applications 5,6. As a result, our group developed a triple-tuned volume coil that uses purely geometric decoupling using only two structures. This addresses the temporal limitations of switching coils while attempting to minimize the losses of common multinuclear coil architectures.Methods
The coil system was developed for imaging at 4.7T. It was composed of a pair of interleaved single tuned nine leg birdcages creating antiparallel fields for hydrogen (200MHz) and sodium (53MHz) and a single tuned phosphorous (81MHz) saddle coil creating orthogonal fields to the birdcages (Fig. 1). The birdcages were constructed with a single structure design on a 17.8cm acrylic former as previously reported7. This birdcage uses two interleaved nine leg birdcages to create linear antiparallel fields for hydrogen and sodium. The saddle coil was designed to nest inside of the birdcage pair with an orthogonal field using adhesive copper tape on a 12.7cm diameter acrylic tube with a 17cm length. The coil was populated with fixed capacitors (1111C Series, Passive Plus) to resonate at approximately 81MHz before variable capacitors (NMAT 40HVE 1712, Voltronics Corp.) were added to match and tune the coil to the precise frequency. The coil was then positioned within the birdcage pair and the rotation angles were adjusted to create orthogonal field angles by minimizing the S21 coupling between the saddle and both ports of the birdcages. Once the decoupling angle had been precisely determined the saddle coil was locked into position via a set of nylon bolts to ensure the decoupling angle did not change throughout the experiments.Results
Dimensioned photographs of the individual and combined coils are shown in Figure 2. The matching of the coils was better than -21.7dB for all three nuclei of interest both combined and separately (Table 1). Q measurements were taken both with separate structures and with the combined triple tuned coil system to show the drop in Q going from the double tuned to the triple tuned structure (Table 1). Decoupling measurements were recorded as an S21 between any given two ports at both frequencies of interest, as shown in Table 2. Decoupling was found to be no worse than -15.4dB. Finally, the fields at all three frequencies of interest were mapped for the triple tuned structure (Fig. 3).Discussion
The interleaved nine-leg birdcages combined with a nested geometric decoupling architecture allowed for a straightforward approach to creating a triple tuned coil. As expected of a triple tuned coil there was a drop in Q, especially for sodium, indicating room for optimization of the design for better coil sensitivity. The S21 decoupling measurements also showed the worst coupling at 200MHz (-15.4dB to the phosphorus coil) with the best decoupling to both coils at 81MHz of -30.2dB. Experimental data indicates that the proximity of the birdcage end rings to the concentric traces of the saddle coils significantly affects the coupling. These dimensions were adjusted in the design process of this coil but left room for further optimization. Modeling of these coupling affects should elucidate the complex coupling paths and allow for improved geometric decoupling of the saddle from the birdcage pair.Conclusion
A triple tuned purely geometrically decoupled volume coil was presented using separate single tuned structures. The purely geometric decoupling technique provided a straightforward decoupling method while minimizing the losses in sensitivity typical of triple tuned coils. Given the relative low sensitivity of large volume coils the optimization of Q is necessary. While sacrificing the sensitivity afforded by quadrature excitation, the coil benefits significantly from the simplicity of using purely geometric decoupling. In the end the desired application and design considerations will need to be balanced to create the ideal coil. Further optimizations to decoupling are needed including the possibility of adjusting saddle coil dimensions as supported by modeling. Following these further optimizations imaging and spectroscopy experiments will be conducted to further validate the viability of this design.Acknowledgements
The authors gratefully acknowledge funding for this project provided by NIH grant number R21EB028516.References
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5. Rao, M. & Wild, J. M. RF instrumentation for same-breath triple nuclear lung MR imaging of 1H and hyperpolarized 3He and 129Xe at 1.5T. 75, 1841-1848, doi:https://doi.org/10.1002/mrm.25680 (2016).
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7. Busher, J. et al. in Proceedings of the 2021 ISMRM & SMRT annual meeting and exhibition.
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